Pens used for ink-jet printing generally include an ink supply housed within the pen. The ink supply is in fluid communication with a set of orifices formed in an orifice plate that is mounted to the pen. A print head mechanism is controlled for forcing drops of ink through the orifice set as the pen is moved relative to a printing medium, such as paper. One such print head mechanism, known as a thermal bubble-type, includes a thin-film resistor associated with each orifice. The resistor is heated to cause sudden vaporization of a small amount of the ink near an orifice. The rapid expansion of the ink vapor forces an ink drop through the orifice.
The ink supply for ink-jet pens is normally stored in a manner such that ink does not leak out of the orifices whenever the print head mechanism is inactive. In this regard, the ink may be stored in a compartment filled with open-cell foam so that the capillarity of the foam prevents the flow of ink out of orifices in the absence of the force generated by the activated print head mechanism for expelling drops.
The mechanism for storing ink so that the ink does not leak from orifices when the print head mechanism is inactive may be different from one pen design to another. Irrespective of the particular storage mechanism used, however, there will be established in the ink storage or supply compartment a back pressure sufficient for resisting ink leakage whenever the print head mechanism is inactive. This back pressure holds ink at the orifice plate to define in each orifice an ink/air interface near the outer surface of the orifice plate. The print head mechanism overcomes the back pressure in ejecting ink from the orifices.
An ink-jet pen, such as the thermal bubble-type just described, will not operate properly unless there exists between the ink supply compartment and the orifice set a continuous path of ink. Put another way, a filled pen must be primed by forcing ink from the supply compartment to the orifice set to eliminate any air that may be present between the ink supply and the orifice set.
A conventional way to prime an ink-jet pen is to place the orifice set in fluid communication with a vacuum source for a time sufficient for drawing ink from the supply compartment and out through the orifice set.
Multi-color ink-jet pens typically include three ink supply compartments, each compartment storing a specific color of ink. Each ink supply compartment is in fluid communication with an associated set of orifices so that one set of orifices ejects ink drops of a specific color. The three ink colors may be, for example, cyan, yellow, and magenta, and may be selectively applied to a printing medium to generate any of a multitude of colors through the process of subtractive color mixing.
In order to ensure reliable color printing, it is important that an ink color associated with one orifice set does not mix with the colored ink of another orifice set prior to reaching the printing medium. Such undesirable mixing of one ink color with another ink color is likely to occur as a result of the priming process mentioned above. In this regard, some of the ink drawn from an ink supply compartment through an associated orifice set during priming may move out of the orifice set and flow along the outer surface of the orifice plate to a location near an orifice set of another color ink. When the priming force (suction) is removed, ink of one color may be drawn into the adjacent orifice set of another color by the back pressure established in the ink supply compartment.
When two colors of ink mix in a single orifice set and are thereafter ejected by the print head, the resultant printed color is not that specified to the printer. Consequently, color print quality suffers until all of the mixed ink is ejected from the orifice set.
In the past, the orifice sets of multi-color ink-jet pens were spaced far enough apart so that during the priming process ink of one color was unlikely to flow along the orifice plate outer surface for a distance sufficient to contact and mix with another color ink.
A prior art ink-jet pen and priming mechanism is depicted schematically in FIG. 1 and includes a multi-color ink-jet pen 20 divided into a cyan-ink supply compartment 22, a magenta-ink supply compartment 24, and a yellow-ink supply compartment 26.
The cyan-ink compartment 22 supplies ink to an associated orifice set 28 formed in an orifice plate 40 that is mounted to the underside of the pen 20. The magenta-ink compartment 24 supplies ink to a second orifice set 30 in plate 40. The yellow-ink compartment 26 supplies ink to a third orifice set 32 in plate 40. The ink in each supply compartment is stored in an open-cell foam medium 27, which medium provides capillarity sufficient for preventing ink from leaking through orifices whenever a print head mechanism associated with each orifice (not shown) is inactive. Accordingly, the capillarity of the foam in each supply compartment 22, 24, 26 establishes a back pressure sufficient for preventing leakage of ink through the associated orifice sets 28, 30, 32.
The prior priming apparatus 34 (FIG. 1) comprises a flexible connector member 36 that is movable against the outer surface 38 of the orifice plate 40. The connector member 36 is shaped to define a substantially sealed priming chamber 42. The priming chamber 42 is placed in fluid communication with a vacuum source 44. Once each ink compartment 22, 24, 26 is filled with ink, the connector member 36 is moved against the orifice plate 40 and a partial vacuum is established within the priming chamber 42 by the vacuum source 44. Ink is, therefore, drawn from each ink supply compartment through an associated orifice set 28, 30, 32. Ink drawn out of an orifice set flows toward the vacuum source 42 to be trapped and disposed of. After a time sufficient for drawing the ink in each compartment through each associated orifice set, the vacuum is removed and the back pressure within each supply compartment thereafter prevents ink from leaking from the orifice sets.
As mentioned above, prior ink-jet pen designs are such that the minimum distance "d" (FIG. 1) between any two orifice sets is great enough so that any ink residing on the outer surface 38 of the orifice plate 40 after the priming process is unlikely to flow the distance "d" to an adjacent orifice set and cause the undesirable mixing mentioned above. As an added measure for preventing printing problems where mixing may occur, the pen is operated for a short time to expel any mixed ink before the pen is packaged for sale.
Current designs of orifice plates for multi-color ink-jet pens have substantially reduced the minimum distance between orifice sets. Accordingly, the likelihood of color mixing as a result of the priming process is increased since residual ink on the orifice plate outer surface need travel only a short distance before mixing with a color of an adjacent orifice set. Expelling mixed ink prior to pen packaging is unacceptable because of the attendant waste of ink. Moreover, mixed ink tends to rapidly disperse into the ink supply, thereby making it difficult to efficiently expel all of the mixed ink prior to packaging.